In web handling operations, curl is often present in multi-layered webs. Curl is defined as the tendency of a web to deviate from a generally flat or planar orientation when there are no external forces on the web. In multi-layered web systems, the curl can be controlled by carefully matching the strains of the webs being laminated together. In products that are direct-coated, such strain matching is much more complicated.
Curl can be controlled in laminated multi-layer webs by carefully matching the strains of the incoming webs. Curl is more difficult to control in direct-coated products, especially where backings are placed under high tension and temperatures, resulting in large strains, while the coating cures at near zero strain. If the induced strain from tension, temperature and cure shrinkage is not matched between the layers, the final product will not lie flat.
Flexing is a process that is used in the process of manufacturing abrasives. Flexing cracks the make-mineral-size coating in the abrasive article. This process makes the abrasive product flexible and reduces the propensity to curl. Sliding the (uncoated) backside of the abrasive over a small radius or pressing abrasive into a rubber roller using a small rotating bar are common flexing techniques. These techniques work very well in the common cases where the product tends to curl toward the abrasive side. These techniques can't be used with the abrasive coated on the contact side because of product damage and tool wear.
Polymer backed abrasive products will have a propensity to curl toward the backing side when direct coated. Minimum line tensions and cure temperatures along with maximum cure shrinkage and backing modulus can help minimize curl problems, but have limitations. If such optimization still results in unacceptable product curl, excess tensile strain will need to be removed from the backing. This could be done with thermal stress relief or by mechanically yielding the backing. Bending the backing around the outside of a small radius on an object will stress the backing to its yield point, causing permanent elongation in the backing.